1. Field of the Invention
The present invention relates to a method and apparatus for inspecting a target by tera-hertz wave spectrometry.
2. Description of the Related Art
A frequency of a far infrared ray or submillimeter wave which ranges about from 0.5 to 3 THz is positioned at the boundary between the frequency range of a light wave and the frequency range of a radio wave. The technology for a light wave and the technology for a radio wave have been advanced respectively. On the other hand, the technology for a far infrared ray or submillimeter wave has not been cultivated from the standpoint of technical advancement and its technical application. However, recently, it is becoming more important to effectively use the frequency range of about 0.5 to 3 THz in radio communication, apply this frequency range to ultrahigh-speed communication, and take advantage of characteristics of an electromagnetic wave of this frequency region for inspecting environment by imaging or tomography. In the following, this frequency region (about 0.5 to 3 THz) of a far infrared ray or submillimeter wave is referred to as a tera-hertz wave.
Patent Literatures 1 and 2, for example discloses means for generating a tera-hertz wave. Non-Patent Literatures 1, 2 and 3 are other documents related to a tera-hertz wave.
[Non-Patent Literature 1]
S. Kawata, K. Sasaki, and S. Minami, “Component analysis of spatial and spectral patterns in multispectral images. I.Basis,” J.Opt.Soc.Am.A4, 2101(1987).
[Non-Patent Literature 2]
K. Sasaki, S. Kawata, and S. Minami,“Component analysis of spatial and spectral patterns in multispectral images. II.Entropy minimization,” J.Opt.Soc.Am.A6, 73(1989).
[Non-Patent Literature 3]
S. Kawata, and S. Minami, “Image Process for Scientific Instrument”, Chapter 11, Color Image and Image Spectroscopy, Publisher CQ, P 259-265.
[Non-Patent Literature 4]
Izumi Yoshiharu, “Guidance for Equipment Analysis”, Chapter 1, Infrared Absorption Spectrum Method”, Kagaku Dojin P 1-20.
[Patent Literature 1]
Japanese Laid-Open Patent Publication No. 2002-72269
[Patent Literature 2]
Japanese Laid-Open Patent Publication No. 2003-5238
One of characteristics of a tera-hertz wave is that the wavelength is not only within the range of the shortest wavelength of a radio wave that propagates through a material, but also within the region of the longest wavelength of a light wave that propagates straight. In other words, a tera-hertz wave can penetrates through various materials like a radio wave, and provide the highest space resolution in a radio wave, and be guided by a lens or mirror like a light wave.
Thus, a tera-hertz wave can penetrates through a semiconductor, plastic, paper, rubber, vinyl, wood, textile, ceramics, concrete, a tooth, a bone, fat, dried food, ice, and so on. For this reason, a tera-hertz wave is expected as imaging means that is an alternative to a X-ray and is safe for a human body.
Recently, one of a terrorist act that anthrax bacteria or drug is distributed by mail was raised as a social problem. The shape of an object enclosed in an envelope can be determined by conventional X ray, but the property of the object in the envelope cannot be determined unless the envelope is opened. Accordingly, when powdery anthrax bacteria or drug is enclosed in an envelope, any abnormality cannot be detected by X ray photograph.